A 7.5-mV-Input Boost Converter for Thermal Energy Harvesting With 11-mV Self-Startup

A DC-DC boost converter for ultra-low-voltage thermal energy harvesting is presented herein. The focus of this brief is to provide both self-startup and efficient conversion at ultra-low input voltages. The converter startup is achieved through the use of a cold starter based on an ultra-low-voltage oscillator and a charge pump. The inductive boost converter architecture uses two low-side switches in order to independently optimize the low-voltage startup and the steady-state conversion efficiency. The model of the developed converter allows optimization of the boost switches as a function of the TEG internal resistance and input voltage. A zero current switching circuit with accurate detection for low input voltages is designed and implemented. The prototype fabricated in 130 nm standard CMOS technology achieves startup for an input voltage of 11 mV and, once started, the system can perform the boost operation for input voltages as low as 7.3 mV, delivering 50% of end-to-end efficiency at 10.5 mV and peak end-to-end efficiency of 85% at 140 mV.

I. Introduction

Thermal energy harvesting is a promising solution to power on-body devices, enabling autonomous consumer and biomedical applications, such as wearable electronics, body area networks, hearing implants, wireless patient monitoring and diagnostics. Harvesting thermal energy from the human body imposes challenges related to the low temperature gradients involved, which are typically of the order of a few degrees Celsius. Typical values of the Seebeck coefficient of commercial thermoelectric generators (TEGs) translate into TEG output voltages of only 10 mV to 30 mV for a temperature gradient of when the available power () is harvested. For common applications where on-body devices are used for patient or athlete monitoring or other Internet of Things (IoT) applications, it is desired that such devices operate uninterruptedly under the thermal gradients provided between the human body and the environment, not requiring a higher thermal gradient for initialization or for cycling between idle and active states.